Difference Between dB and dBm

When a physical quantity, such as power or intensity, is measured relative to a reference level it is expressed in decibels (dB), which is a logarithmic unit. Decibel is considered as a dimensionless unit because it is a ratio of two quantities with the same unit thus cancellation takes place. It is used for quantifying the ratio between two values. The best example of this is the signal-to-noise ratio.

Sound pressure level is typically measured in dB but the unit is not limited to that quantity alone. There are a lot of uses of this measuring unit particularly in engineering. Since it is applicable in measuring signals, anything that can be expressed in waves may also be measured with dB. In the disciplines of acoustics electronics, dB is liberally utilized.

To be exact, decibel dB is expressed in this term: dB = 10 log (P1/P2). Where P1 and P2 are two different values of power.

It is primarily used because it can represent an extremely huge number into a convenient scale. In radio link designs, values often differ enormously and to contrast these values decibel is used. Its logarithmic properties make calculation easier. With the implementation of dB, engineers and physicist are now able to calculate values with simple few digit numbers as an alternative of arduous 9 to 10-digit ones.

dBm is different but definitely related to dB. dBm stands for an absolute power level. It is in reference to another unit of power the milliwatt.

Mathematically, dBm = 10 * log (P/1mW)

The value of “P” is power in watts. Then, with further calculation, you can convert the absolute power unit “P” into dBm. The value of power level “P” is now referenced to 1 mW. The unit dBm is devised because in practice, 1 mW is a convenient reference point from which to measure power. dBm is considered as an absolute unit ‘“ a unit to measure power.

Additionally, based on what value the power is referred to, a particular absolute value of power can be in any kind. If dBm ‘“ which can be written in dBmW by the way — is acquired because of 1 mW reference, a value can be in a form of dBW if it is referred to 1 watt.

Summary:

1. dB is used to quantify ratio between two intensity or power values while dBm is used to express an absolute value of power.

2. dB is a dimensionless unit while dBm is an absolute unit.

3. dB is relative often relative to the power of the input signal while dBm is always relative to 1 mW signal.

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Written by : Ian.
and updated on October 11, 2011

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